The invention pertains to the art of thermal dilution techniques and more particularly to the so called local thermodilution technique for determination of venous blood flow.
Venous blood outflow of several organs such as the heart, kidneys and brain have been variously measured by what is generally termed local thermal dilution techniques. All thermal methods for the study of circulation depend upon the induction of a change in the heat content of the blood stream. As a result of this change, a particular time dependent spacial distribution of temperature is developed in the blood according to the manner in which the change is brought about and as a function of the prevailing pattern of blood flow. Generally, thermal dilution refers to those techniques in which, following the artificial induction of a known change in the blood heat content at some point in the circulation, the resulting temperature change is followed at such a distance downstream that an even distribution of heat is presumed to have been developed over the whole vascular cross-section. Insofar as the specific technique with which the subject application is concerned, a catheter is employed to inject a mass into the blood stream which is miscible with and at a different temperature than the blood. This technique was first introduced in 1953 as a method for the measurement of volumetric blood flow rate and was termed thermodilution. Following initial introduction of the thermodilution technique on measurement of cardiac output, it was extended to the measurement of flow in single blood vessels in other parts of the body.
The so called thermodilution technique as just above generally described is now well known and widely used in the medical field. In practicing this technique, a wide variety of types and styles of catheters have been variously employed. Basically, these catheters include means for injecting the liquid injectate into the blood stream and means for sensing the temperature of the blood stream downstream of the point of injection. Such temperature sensing means may comprise either thermocouples or thermisters. In typical application, a charge of cold liquid is injected into the blood stream through the catheter in such a way as to produce intimate comixture following which the time course of temperature change is recorded through the thermocouple or thermister at a suitable point downstream in the blood flow. From this temperature-time curve and knowledge of the magnitude of the change in heat content of the blood produced by the injecting, the volume rate of flow can be calculated in a known manner.
The practicality of using the thermodilution technique insofar as its simplicity, the possibility of repeated measurements at short intervals and the use of a harmless indicator has been limited to a considerable extent by the complexity in construction of most thermodilution catheters heretofore available. Those catheter designs that were kept simple necessarily had to be of a large size and thus not suitable for many vessels. Moreover, all have shared certain undesirable characteristics which detracted somewhat from the validity of the results obtained in use.
That is, and in addition to the usual requirements such as adequate injectate mixing, negligible heat dispersion and adequate distance of the temperature sensing means from the injection site required to achieve acceptable results, two conditions related to the catheter itself must also be met. First, there must be absolute thermal insulation of the temperature sensing means with respect to the injectate and second, there must be a constant relative position between the injection and sensing sites with regard to the dynamics of flow and temperature changes in a vessel. These two requirements are deemed indispensible for accurate and reproducible determinations, the first because lack of insulation leads to unstable base line and distorted curves and the second because the geometry of heat dispersion influences the curves.
The importance of the above noted conditions is generally accepted and acknowledged by many investigators and the approaches heretofore suggested for meeting them have either been complex or unreliable. Prior catheter designs have included complex maneuvers utilizing umbrella catheters, separate wires and air insulation which have not proved to be particularly successful.
The present invention contemplates new and improved apparatus and method which overcome all of the above referred to problems and others to provide a new thermodilution catheter and method which are simple in design, easy to use, economical to use and which are readily adaptable to use in a number of different large and small blood vessels when practicing the local thermodilution technique.